Without limiting the scope of the present invention, its background will be described with reference to perforating a hydrocarbon bearing subterranean formation with a shaped charge perforating gun apparatus, as an example. After drilling a section of a subterranean wellbore that traverses a hydrocarbon bearing subterranean formation, individual lengths of metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path through which fluids from the formation may be produced to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string or to place addition cement behind the casing string, hydraulic openings or perforations must be made through the casing string and a distance into the formation.
Typically, these perforations are created by detonating a series of shaped charges located within one or more perforating guns that are deployed within the casing string to a position adjacent to the desired location. Conventionally, the perforating guns are formed from a closed, fluid-tight hollow carrier gun body adapted to be lowered into the wellbore on a conveyance such as coiled tubing, a jointed tubing or the like. Disposed within the hollow carrier gun body is a charge holder that supports and positions the shaped charges in a selected spatial distribution. The shaped charges have conically constrained explosive material therein. A detonating cord that is used to detonate the shaped charges is positioned adjacent to the initiation ends of the shaped charges. The detonating cord is typically activated by a firing head when it is desired to initiate the perforating guns.
Many conventional firing heads are operated in response to pressure applied to the firing head from a remote location. For example, many pressure operated firing heads rely on shear pins to select a pressure which, when applied to the firing head, shears the pins and initiates the detonation sequence. With pressure actuated firing heads, the pressure required to trigger actuation must typically be the highest pressure required to trigger actuation of any pressure actuated component in the well. It has been found, however, that the perforation event may require a wellbore pressure that is not consistent with the actuation pressure of the firing head. Depending upon the particular design of the completion, it may be desired to create an underbalanced pressure condition in the wellbore, a balanced pressure condition in the wellbore or a particular overbalanced pressure condition in the wellbore prior to the perforation event. Accordingly, pressure in the wellbore must be reduced after the pressure event that actuates the firing head but before the perforation event.
Efforts have been made to overcome this pressure balance issue using time delay devices, which may be added to a firing head to extend the time period between the pressure event and the perforation event. Convention time delay devices use pyrotechnic time delay fuses that provide delays in the order of minutes. To create a longer delay, more than one pyrotechnic time delay fuse may be added to the firing head. It has been found, however, that in certain installations wherein a time delay in the order of hours is desired, the number of pyrotechnic time delay fuses required the achieve such a time delay and the connections required between the pyrotechnic time delay fuses make these systems unreliable. In addition, the length of a system of pyrotechnic time delay fuses needed to achieve such a time delay makes such a system unfeasible.
A need has therefore arisen for an improved firing head that is operable to provide a time delay between the pressure event and the perforation event. In addition, a need has arisen for such an improved firing head that does not require numerous time delay elements to provide sufficient time for pressure balancing the well prior to the perforation event. Further, a need has arisen for such an improved firing head that does not require time delay elements having an unfeasible length to provide sufficient time for pressure balancing the well prior to the perforation event.